6 3 6 



CONSTANTS OF MAGNETISATION. 



numbers of the same order. Such differences may be partially 

 due to defects of the method of observation, but they are prin- 

 cipally due to the demagnetising action of cylindrical rods. This 

 action only becomes negligable provided the ratio of the length 

 of the cylinder to its diameter is extremely great; it is recognised 

 if the two methods are simultaneously used. 



Iron rods magnetised by a long cylindrical coil are at the 

 same time surrounded in the centre by a short coil of p spires, 

 which communicates with a ballistic galvanometer. The magnetic 

 moment M is determined by the deflection of a declinometer, and 



A o 



c 9 



by the discharge Q corresponding to the reversal of the magnetising 

 current I. 



The quotient of the magnetic moment M by the volume of 

 the rod gives its mean magnetisation A. On the other hand, 

 if A 1 is the magnetisation of the median section, F x the mag- 

 netising action, and S the section of the rod, the discharge Q 

 gives the product 



2ps 



- Fj) = 8/7rS 



F 



! - = 8/;rSA' . 

 4 73 



The ratios of the quantities A and A' to the magnetising force 

 give two coefficients / and /' which have not the same 

 meaning, but should be equal to each other and to the coefficient 



